The present invention relates generally to reducing fan noise, and more particularly to noise reduction in a fan assembly having two fans driven by a single fan motor.
Fans are used typically for cooling purposes but may be used for other purposes such as, but not limited to, producing thrust in a turbofan jet engine. Cooling fans, such as conventional axial and centrifugal fans, are used on a large number of appliances and electronic and electrical equipment. Noise from these fans is often a problem when the equipment is designed to operate near people, such as in a home or in an office environment. For example, microwave ovens, including conventional designs having a ducted fan assembly with two fans driven by a single fan motor, produce unwanted noise in the kitchen. Fan noise typically has two components: a narrowband component which is due to interactions between the fan's rotating blades and fixed parts, and a broadband component which is due to flow turbulence as the air passes through the fan. The narrowband noise consists of a tone at the blade passage frequency and its harmonics. It is a deterministic signal. The broadband noise is random and is spread over a wide frequency band. Broadband noise tends to be less intrusive to a listener, and conventional techniques for reducing fan broadband noise include adding a resonator, a muffler, or sound deadening material. The narrowband tonal noise is usually more annoying since the listener tends to key on single frequency tones. Conventional techniques for reducing fan narrowband tonal noise include adding a resonator or a muffler, using ducts lined with sound deadening material, and employing active noise control methods. What is needed is a fan design which generally eliminates the annoying narrowband tonal fan noise.